Ruby Ford is a major crossing of the River Trident, located just east of the point where the Green, Blue, and Red Forks of the Trident converge into a single river, and the waters are thus slow enough to cross.
Armies moving north or south of the Trident river system (which separates both the North and the Vale of Arryn from the rest of Westeros) are basically faced with two choices for their route. The first choice is to travel along the lighter headwaters of the three forks of the Trident, though this does necessitate crossing the Green Fork at The Twins castle ruled by House Frey. The other option is to march east and use the Ruby Ford. The Kingsroad itself, extending between King's Landing and Winterfell, crosses the Trident at the Ruby Ford. East of the Ruby Ford the combined size of the single river, now fed by all three tributaries, makes it too wide and deep to cross. This makes the Ruby Ford of vital strategic importance.
The Ruby Ford gained its name in the climactic Battle of the Trident during Robert's Rebellion, when Robert Baratheon killed Prince Rhaegar Targaryen with his warhammer, and the death-blow struck Rhaegar so forcefully that the many rubies that ornamented his breastplate were shattered off and were scattered in the river.
In the Books
It is unknown what specific name, if any, the ford had in the thousands of years that it was used before the Battle of the Trident. One of the reasons Arya Stark wandered away from camp in the part of the books corresponding to "The Kingsroad", apart from wanting to practice swordfighting with Mycah, is because she wanted to search along the banks of the Ruby Ford for the rubies for which it is named. Sansa Stark insisted to her sister that in the many years since Rhaegar's death, all of the rubies have already been found.